Solid state light emitting devices, such as light emitting diodes (LEDs) and laser diodes, are widely used for the generation of light in a wide variety of applications including, for example, displays and lighting, as they represent an efficient light emitting structure.
One example of an approach to manufacturing light emitting diodes is taught by US2010/0283080. A light emitting diode die is formed by depositing a stack of p-, i- and n-type semiconductor layers on a substrate. The active, light emitting layer is the i-type layer. Electrons and holes introduced from the p-type and n-type layers produce light in the active i-type layer sandwiched between the n-type and p-type layers.
Those skilled in the art will realize that a wide variety of semiconductor structures may be used as an alternative to the p-i-n semiconductor stack of layers.
A number of metallic layers may be formed over the stack with a variety of purposes. One layer may be an ohmic contact, providing a good contact to the upper semiconductor layer. For example, US2010/0283080 suggests an ohmic contact layer of Ni, Ag or Pd as an ohmic contact to the upper p-type semiconductor layer. This ohmic contact layer may be thin.
It is also useful to provide a reflective layer to reflect light back and this may be formed as a separate layer of Ag, for example, deposited over the ohmic contact layer.
Other metallization layers may also be provided such as a guard layer. In US2010/0283080 the guard layer is a multilayer of TiW, TiW:N and TiW.
In order to more efficiently manufacture a plurality of diodes, multiple devices are formed on a single substrate and these are then separated. To prepare for such separation, US2010/0283080 teaches forming trenches through the metallization layer and into the semiconductor layers along streets along which the separation will take place at a later stage.
An insulator of dielectric may then be formed in the trenches.
Further metal layers, for example connections to the n-type semiconductor layer, and/or further layers of dielectric may then be formed.
After forming these layers on the substrate, the substrate may be divided into a plurality of dies along the trenches. Thus, the trenches formed earlier in the process function as streets to divide the substrate into individual light emitting diodes each as a separate die. There are a number of different approaches to the separation step, for example scribing, sawing or laser cutting.
The individual dies can then be mounted in a package.
The step of dividing the substrate into individual dies and the packaging process can cause a considerable amount of stress.
The various different metals used for different purposes in the layers of the metallization frequently have a relatively low adhesion with each other and with the semiconductor layers and dielectric. Accordingly, during the step of separation of the dies the resulting stress can cause the layers to separate from one another in an effect known as die edge delamination.
Another effect of the stress may be cracking of one or more of the layers. This can be a particular problem in the dielectric, though other layers may also be affected.
There is thus a need for a manufacturing process and a light emitting diode that takes account of the effects of stress.
WO2013/033685A1 discloses LED dies on an LED substrate wherein singulation is performed to provide multiple LED dies that are joined to a single carrier die. The multiple LED dies on the single carrier die are connected in series and/or in parallel by interconnection in the LED dies and/or in the single carrier die. A crack reducing interlayer is provided on an insulation layer capable of reducing the propagation of cracks through the insulation layer.
EP255259A1 discloses semiconductor light emitting device in which adhesion between an insulating layer and a semiconductor layer is improved while maintaining the ability of the insulating layer to limit the direction of current flow. A metal layer is provided on the semiconductor layer having a higher adhesion with the semiconductor layer than the insulating layer. The metal layer is provided in a shape of islands and the insulating layer fills gaps between these islands.
US2012/199861A1 discloses a semiconductor light emitting element including a light emitting layer and a p-type semiconductor layer laminated on an n-type semiconductor layer, a transparent conductive layer laminated on the p-type semiconductor layer, a transparent insulating layer laminated on the transparent conductive layer and the exposed n-type semiconductor layer, the transparent insulating layer having plural tapered through-holes formed therein, a p-electrode formed on the transparent conductive layer with the transparent insulating layer interposed therebetween, the p-electrode being connected to the transparent conductive layer via the through-holes provided for the transparent insulating layer; and an n-electrode formed on the n-type semiconductor layer with the transparent insulating layer interposed therebetween, the n-electrode being connected to the n-type semiconductor layer via the through-holes provided for the transparent insulating layer.